Optical communications systems as presently contemplated will have a light source and a photodetector optically coupled to each other by means of a glass transmission line which is commonly referred to as an optical fiber. The light source is typically a semiconductor device such as a light emitting diode or a laser. Optical fibers presently used in such communications systems are based on silica compositions which have a region of lowest loss due to absorption and scattering at approximately 1.5 .mu.m and a region of essentially zero dispersion at approximately 1.3 .mu.m. The latter region is of interest because it minimizes the problems associated with pulse broadening due to, for example, material dispersion. Both wavelength regions are desirable for optical communications systems use because they permit, for example, a greater spacing between repeaters or higher information transmission rates. Repeaters are components of the system which detect and regenerate the optical signals. Accordingly, lasers capable of operating in either or both of these wavelength regions are of great interest for optical communications systems.
The present semiconductor laser structure of choice for optical communications systems is a single or double heterostructure laser. Several materials systems, such as Group III-V or Group II-VI compound semiconductors, can be used within these two spectral regions mentioned above to fabricate such lasers. One materials system of great interest for lasers is the quaternary Group III-V alloy In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y which is epitaxially grown lattice matched to InP substrates. However, use of this alloy has several drawbacks. For example, control of the P to As atomic fraction ratio is often difficult when the quaternary alloy is grown by molecular beam epitaxy or vapor phase epitaxy because of the memory effect of the growth system for high vapor pressure Group V elements. Additionally, P is generally a difficult element to deal with and it would be desirable to avoid its presence. Another possible materials system for lasers is the quaternary Group III-V alloy, Ga.sub.x Al.sub.y In.sub.1-x-y As, which is also grown lattice matched to InP substrates. If the subscripts x and y are adjusted so as to maintain lattice matching to InP, i.e., x+y=0.47, the bandgap of this quaternary alloy can be varied from 1.47 eV to 0.76 eV, thereby permitting the wavelength range from approximately 0.87 .mu.m to approximately 1.65 .mu.m to be covered. This system has the advantage that it avoids the problems that arise when P is used. However, in general, growth of device structures is easier if the semiconductors are binary or ternary rather than quaternary. Additionally, InAlGaAs lasers emitting near 1.5 .mu.m have relatively low luminescence efficiency and high current thresholds. It is hypothesized that these effects are due to a large density of non-radiative centers associated with the presence of Al in the active layer.
In addition to varying the composition to vary the wavelength of emitted radiation, there is another approach to obtaining wavelength tuning within a given materials system. This approach involves the growth of what are now termed "multi-quantum well lasers." Such lasers are described in U.S. Pat. No. 3,982,207 issued on Sept. 21, 1976 to Raymond Dingle and Charles H. Henry. The active region in these lasers is formed by alternating layers of relatively low bandgap material and layers of relatively high bandgap material. In a particular embodiment described, the active low bandgap layers comprised Al.sub.r Ga.sub.1-r As and the passive high bandgap layers comprised Al.sub.z Ga.sub.1-z As with r&lt;z. Other systems materials, such as AlGaAsP, were mentioned. The bandgaps are described, with respect to each other, as being either "low" or "high." The former layers are termed "well layers" and the latter layers are termed "barrier layers." All layers in the active region are thin to ensure that quantum size effects become important in determining the wavelength of emitted radiation. The carriers are confined to the well layers by the barrier layers and, in the first approximation, the carriers may be viewed as particles in a one-dimensional infinite potential well. As is well known from elementary quantum mechanics, see, for example, Quantum Mechanics, L. I. Schiff, pp. 37-44, 1968, the energy levels of particles in such are as given by: ##EQU1## where h is Planck's constant divided by 2.pi., m* is the effective mass of the carriers, L is the width of the well layer, and n is a positive integer. The emission energy thus increases as the inverse square of the width of the relatively low bandgap material and inversely with the carrier effective mass.
Fabrication of multi-quantum well lasers is more difficult than is the fabrication of, for example, double heterostructure lasers for at least several reasons. First, the presence of a plurality of well and barrier layers means that there will be more interfaces than there are in more conventional laser structures. These interfaces need to be of high quality if the structure is to operate efficiently as a laser. Second, all of the epitaxial layers should be lattice matched to each other. Satisfaction of this condition with, for example, AlGaAs, is relatively easy because the lattice constant is relatively constant over a wide range of compositions. However, for other materials systems in which the lattice constant varies significantly with composition, the compositions of the well and barrier layers have to be carefully controlled if they are to be lattice matched.
However, multi-quantum well lasers emitting precisely selected wavelengths near at approximately 1.5 .mu.m have not been demonstrated and, in fact, cannot be demonstrated with AlGaAs.